Items that are shipped by air typically are loaded first onto specially configured pallets or into specially configured containers. In the airfreight industry, these various pallets and containers commonly are referred to as Unit Load Devices (“ULDs”). ULDs are available in various sizes, shapes and capacities.
A ULD typically is loaded with cargo at a location other than the immediate vicinity of an aircraft. Once a ULD is loaded with cargo items, the ULD is weighed, transferred to the aircraft, and is loaded onto an aircraft through a doorway or hatch using a conveyor ramp, scissor lift, or the like. Once inside the aircraft, a ULD is moved within the cargo compartment to its final stowage position. Multiple ULDs are brought onboard the aircraft, and each is placed in its respective stowed position. Once the aircraft reaches its destination, the ULDs are unloaded from the aircraft in a manner that is the reverse of the loading procedure.
To facilitate movement of a ULD within an aircraft cargo compartment as the ULD is loaded, stowed, and unloaded, the deck of an aircraft cargo compartment typically includes a number of raised roller elements. These roller elements often include elongated roller trays that extend longitudinally along the length of the cargo deck, ball panel units, and the like. For example, roller trays typically include elongated rows of cylindrical rollers that extend in a fore and aft direction. Ball panel units include plates with upwardly protruding rotatable spherical balls. The ULDs sit atop these roller elements, and the roller elements facilitate rolling movement of the ULDs within the cargo compartment. Cargo decks also commonly are equipped with a plurality of power drive units (PDUs). PDUs are electrically powered rollers that can be selectively energized to propel or drive a ULD in a desired direction over a cargo deck's roller elements.
Generally, PDUs can be one of two basic types. A first type of PDU is secured to a cargo deck structure such that the powered drive roller can only rotate in fore and aft directions within a cargo hold. Such a “fixed” PDU typically is installed within a cargo roller tray such that the PDU's drive roller protrudes above a plane defined by the uppermost portions of adjacent roller elements when the drive roller is in an active position. The drive roller can be either an inflated tire or a rigid roller having an elastomeric rim. The rotating tire or roller contacts and grips the bottom of an overlying ULD such that the ULD is driven in a desired direction by traction between the roller and the underside of the ULD. Such stationary PDUs often are configured such that the drive roller can be selectively moved between an active raised position, and a retracted inactive or stowed position. The lifting of the drive roller from the retracted position can be actuated by springs, by an electrically powered lift mechanism, or the like. Such fixed PDU's typically are installed at cargo deck locations remote from an aircraft's cargo door, where a ULD's movement can be substantially limited to the fore and aft directions.
A second type of PDU is known as a “steerable PDU”. In a typical steerable PDU, the drive roller is mounted to a rotatable frame or turntable that can be selectively oriented to align the drive roller in a desired direction within a cargo hold. Like the fixed PDUs described above, a steerable PDU can be configured to lift and retract the drive roller between its active raised position and its inactive retracted position. Steerable PDUs usually are installed at cargo deck locations that are proximate to an aircraft's cargo door, where a ULD may require movement in a direction other than the fore or aft directions as the ULD is being loaded and/or unloaded.
The bottom surfaces of ULDs can be irregular either due to their original construction or due to damage or deformation from prior use. Accordingly, when a ULD with an irregular bottom surface moves over an active PDU, the degree of contact between the unit's drive roller and the ULD can vary between full contact, partial contact, and zero contact. Once contact between the drive roller and the irregular surface of the ULD is lost or substantially reduced, the traction force between the drive roller and the ULD can be lost or reduced. When such lost or reduced traction occurs, the movement of the ULD within the cargo hold can be slowed or stopped, which detrimentally affects the cargo loading or unloading process. Though drive rollers that include resilient inflated tires can accommodate a certain amount of variation in contact between the drive roller and a ULD, non-inflated drive rollers are substantially less compliant to variations in the geometry of a ULD's undersurface.
One solution to this problem of lost or reduced contact and traction between a drive roller 20 and an irregular bottom surface 42 of a ULD 40 is illustrated in FIG. 1. In FIG. 1, a load-compliant PDU lift system 10 includes a drive roller 20 on a drive shaft 22 that is rotatably mounted to a yoke 12. As used herein, the term “load-compliant” means capable of automatically adapting to substantial variations or irregularities in the geometry of the undersurface of a ULD that contacts a PDU's drive roller. A first end 14 of the yoke 12 is pivotally mounted to a base 30 about a pivot axis 18. A second end 16 of the yoke 12 is vertically supported by one or more springs 50 disposed between the second end 16 and the base 30. Accordingly, as the spring is compressed by a vertical load “L” on drive roller 20, the second end 16 of the yoke 12 moves downward, the yoke 12 pivots downward, and the attached drive roller 20 also moves downward. Conversely, as the spring 50 pushes the second end 16 upward, the yoke 12 pivots upward, and the attached drive roller 20 also moves upward. Thus, the spring 50 permits the drive roller 20 to move up and down as necessary to maintain contact with an irregular bottom surface 42 of the ULD 40 as the ULD 40 is propelled by a traction force “FT” applied by the roller 20. The spring (or springs) 50 is sized such that the vertical force applied by the spring 50 is sufficient to maintain frictional contact between the drive roller 20 and the bottom surface of the ULD under load L. The PDU lift system 10 shown in FIG. 1 can be adapted to mount to a stationary support, frame or base 30, or to mount to a steerable rotating support or frame. In addition, the PDU lift system 10 can be configured such that the drive roller 20 is selectively retractable.
Though the load-compliant PDU lift system 10 depicted in FIG. 1 may effectively maintain contact between the drive roller 20 and a ULD's irregular bottom surface 42, such a PDU lift system 10 has some shortcomings. As shown in FIG. 1, the drive roller 20 is selectively operable to be driven and rotate in a counterclockwise driving direction “I”, and to be driven and rotate in an opposite clockwise driving direction “II”. The drive shaft 22 and drive roller 20 are rotated by a drive motor (not shown in FIG. 1) that is affixed to the yoke. When the drive roller 20 is driven in a counterclockwise direction “I” under vertical load L, the drive roller 20 is subjected to a traction force FT (acting left to right in FIG. 1) due to the frictional drag between the drive roller 20 and the bottom surface 42 of the overlying ULD 40. Because the drive roller 20 is connected to a drive motor that is affixed to the yoke 12, this traction force FT results in a clockwise torque TCW acting on the yoke 12 that is equal to the traction force FT times the vertical distance “H” between the top of the drive roller 20 and the yoke pivot axis 18 (TCW=FT·H). The clockwise torque TCW in turn forces the yoke 12 to rotate in a clockwise direction, thus compressing the spring 50, and causing the yoke 12 and drive roller 20 to move away from the ULD 40, and thus lessening the degree of contact between the drive roller 20 and the ULD's bottom surface 42. Because the traction force FT applied to the ULD 40 by the drive roller 20 is dependent upon the degree of contact between the drive roller 20 and the ULD 40, the additional compression of the spring 50 that results from the counterclockwise rotation of the drive roller 20 is detrimental to the magnitude of the driving force FT that is effectively applied to the ULD's bottom surface 42.
In contrast, when the drive roller 20 is driven in a clockwise direction “II”, the direction of the frictional traction force FT is opposite from that shown in FIG. 1 (i.e. right to left in FIG. 1), and the resulting torque TCCW on the yoke 12 thus acts in a counterclockwise direction. This counterclockwise torque TCCW forces the yoke 12 to rotate in a counterclockwise direction about the yoke pivot axis 18 and to move upward, thereby increasing the degree of contact between the drive roller 20 and the underside 42 of the ULD 40. Because the degree of contact between the drive roller 20 and the ULD's underside 42 is increased by this movement, the effective traction force FT on the underside 42 of the ULD 40 is enhanced by the counterclockwise movement of the yoke 12. Thus, the load-compliant PDU 10 has a “strong” driving direction (left to right in FIG. 1), and a “weak” driving direction (right to left in FIG. 1). One possible solution to this problem is to provide larger and stiffer springs 50 to minimize the amount of additional spring compression that results from the traction force FT. But such large springs 50 can undesirably increase the size and weight of the PDU 10.
Accordingly, there is a need for a load-compliant PDU, and more specifically, for a load-compliant PDU lift system that is equally effective in driving ULDs in two opposite directions.